compiler wise, you do not need to specify -mx32 yourself. the toolchain
defaults to the x32 ABI (and all programs in there are compiled as x32). you
only need -mx32 if you want to do something like distcc and execute with
toolchains that aren't targeting x32 by default.
as for what are valid flags, pretty much everything that works with your amd64
target will work with x32. very little is different between them.
-mike